An arc furnace, as is used, for example, for melting steel, generally has a transformer connected upstream of it, which transformer sets an AC voltage required for the arc. Since very high powers are consumed by an arc furnace and high AC voltages need to be transmitted by the transformers connected upstream, such transformers are generally introduced into an insulating material in order to avoid flashovers.
An important characteristic in the case of AC voltage and alternating current is the reactance, i.e. the reactance of a conductor, for example in the case of a coil through which current is flowing.
For various operational states of an arc furnace it is desirable to be able to set different reactances. For this purpose it is known to integrate an apparatus for setting the reactance with an induction coil and with an on-load tap changer in the transformer which is connected upstream. Typically, such apparatuses with the coils and the active part of the transformer are introduced into a tank filled with insulating material.
It is further known for an arc furnace plant without an additional reactance introduced into the transformer tank to use air-core induction coils positioned outside the transformer, for example in an outdoor switchgear assembly, as the additional reactance.
However, it is not possible to use additional reactances designed in this way to set an optimum reactance for all operational states of the arc furnace on load.
Many arc furnace plants are therefore operated in practice with a fixedly preselected reactance. The induction coil used for this purpose has at least one tap, which taps off the current flowing through the coil after a specific turns number and therefore assigns a reactance which is set in a defined manner to the transformer. For assignment of the desired reactance, the tap has fixed wiring. However, it is necessary for a change to be made if it is identified after a relatively long period of operation that the selected series reactance has not been optimally set during operation on load, i.e. during operation of the furnace, which results, for example, in an unnecessary increase in the consumption of energy, or that the reactance should be matched in a process-dependent manner. It is disadvantageously necessary for this purpose for the power supply to be switched off, and therefore for the furnace plant to be shut down and for the transformer to be wired to another tap on the induction coil.